The invention relates generally to re-direction of photon beams, and more particularly to systems and methods for producing high-flux photon beams
Many imaging applications using photon radiation, such as X-ray radiation, require higher flux output to improve the signal-to-noise ratio in the measured data. Increasing X-ray flux may be accomplished, for example, by redirecting into usable directions X-ray radiation emitted by an X-ray source that would otherwise not contribute to the fidelity of the measured data. For example, energy-dispersive X-ray diffraction (EDXRD) may be used to inspect checked airline baggage for the detection of explosive threats or other contraband. Such EDXRD may suffer from high false positives due to poor signal-to-noise ratio in diffracted X-ray signals, which may stem from a variety of origins. First, the polychromatic X-ray spectrum used in EDXRD is produced by the Bremsstrahlung part of the source spectrum, which is inherently low in intensity. Second, X-ray source collimation may eliminate more than 99.99 percent of the source X rays generated, since these X rays are not directed toward the baggage volume under analysis. Third, some of the materials being searched for, e.g., explosives, may not diffract strongly, leading to diffuse peaks in measured X-ray spectra, as the materials are amorphous or polycrystalline. Fourth, the diffracting volume may be small due to the need to improve the resolving capability of the system. Most of these limitations arise from the type and configuration of threat materials being searched for in baggage and the nature of the X ray interaction with these materials, making all but the second limitation unavoidable.
At lower X-ray energies, such as 60 keV and below, increasing the polychromatic X-ray flux density at the material being inspected has been addressed by coupling hollow glass polycapillary optics to low powered, sealed-tube (stationary anode) X-ray sources. The glass comprises the low index of refraction material, and air filling the hollow portions comprises the high index of refraction material. Total internal reflection from these types of optics typically becomes vanishingly small at energy levels above 60 keV, due to non-optimal differences in the real and imaginary parts of the refractive indices of air and glass.
Further, such optics use the concept of total internal reflection to reflect X rays entering the hollow glass capillaries at appropriate angles back into the hollow capillaries, thereby channeling a solid angle of the source X rays into collimated or focused beams at the output of the optic. As used herein, the term “collimate” refers to the creation of approximately parallel beams of electromagnetic (EM) radiation from divergent EM beams. The divergence of the parallel beams is on the order of the critical angle for total internal reflection, which vary from a few mill-radians at around 20 keV to 0.01° and lower for energies above 60 keV. Only about one to two percent of an EM source's solid angle typically is captured by the input of such known optics. In addition, the use of air as the high refractive index material in capillary or polycapillary optics prevents such optics from being placed within a vacuum, limiting their potential uses.
Thus, a device that can collect more of the generated source photons than are currently used and redirect them into a specified volume would be desirable for improving the SNR of a variety of X-ray analysis techniques.